The invention relates to a process for preparing hydrophobic particulate solids containing Si-OH groups and to the use of the resultant hydrophobic particulate solids in compositions containing diorganopolysiloxanes, which can be cured to form elastomers.
U.S. Pat. No. 3,953,487 to Kratel and corresponding DE-OS 2,344,388 discloses that silicon dioxide can be rendered hydrophobic in inert, organic solvents and in a high speed homogenizing and dispersing apparatus driven at over 2000 revolutions per minute. However, at these high rotational speeds, high material wear and tear and abrasion result. Furthermore, 3 to 25 percent by weight of water repellent, based on the solids, are used in this process for rendering solids hydrophobic, which results in long reaction times, a relatively high expenditure of energy and poor production.
According to the prior art, compositions based on diorganopolysiloxanes, which are curable to form elastomers, and containing hydrophobic fillers, are prepared by rendering the filler hydrophobic by the addition of a water repellent during mixing of the filler with the diorganopolysiloxane, i.e. in situ. The use of a hydrophobic filler, which has been rendered hydrophobic in a fluidized bed, pug mill machine or stirred ball mill, etc. such as disclosed in U.S. Pat. No. 3,868,345 to Kratel, or in corresponding DE-OS 2,211,377 in compositions which are curable to form elastomers, is impossible in many cases, since the properties of the products prepared therefrom differ from those of the products produced in the in-situ process. However, the in-situ process has the disadvantages of a long batch cycle time and high emissions which occur in many places and are therefore difficult to control. Moreover, a selective control of the process for rendering fillers hydrophobic is scarcely possible and also, corrections to the filler content of the compositions based on diorganopolysiloxanes, which are curable to form elastomers are no longer possible since suitable fillers are not available.
Therefore, it is an object of the present invention to provide a process for rendering a particulate solid containing Si-OH groups hydrophobic. Another object of the present invention is to provide a process for rendering a particulate solid containing Si-OH groups hydrophobic and to selectively control the degree of water repellency. Still another object of the present invention is to provide a process for rendering a particulate solid containing Si-OH groups hydrophobic having a high and/or uniform degree of water repellency. A further object of the present invention is to provide a process for rendering a particulate solid containing Si-OH groups hydrophobic which may be used in preparing diorganopolysiloxanes which can be cured to form elastomers.